Amorphous alloys (or metallic glasses) have been generally been prepared by rapid quenching from above the melt temperatures to ambient temperatures. Generally, cooling rates of 105° C./sec have been employed to achieve an amorphous structure. However, at such high cooling rates, the heat can not be extracted from thick sections, and, as such, the thickness of articles made from amorphous alloys has been limited to tens of micrometers in at least in one dimension. This limiting dimension is generally referred to as the critical casting thickness, and can be related by heat-flow calculations to the cooling rate (or critical cooling rate) required to form an amorphous phase.
This critical thickness (or critical cooling rate) can also be used as a measure of the processability of an amorphous alloy. Until the early nineties, the processability of amorphous alloys was quite limited, and amorphous alloys were readily available only in powder form or in very thin foils or strips with critical dimensions of less than 100 micrometers. However, in the early nineties, a new class of amorphous alloys was developed that was based mostly on Zr and Ti alloy systems. It was observed that these families of alloys have much lower critical cooling rates of less than 103° C./sec, and in some cases as low as 10° C./sec. Accordingly, it was possible to form articles having much larger critical casting thicknesses of from about 1.0 mm to as large as about 20 mm. As such, these alloys are readily cast and shaped into three-dimensional objects, and are generally referred to as bulk-solidifying amorphous alloys.
Another measure of processability for amorphous alloys can be described by defining a ΔTsc (super-cooled liquid region), which is a relative measure of the stability of the viscous liquid regime of the alloy above the glass transition. ΔTsc is defined as the difference between Tx, the onset temperature of crystallization, and Tsc, the onset temperature of super-cooled liquid region. These values can be conveniently determined by using standard calorimetric techniques such as DSC measurements at 20° C./min. For the purposes of this disclosure, Tg, Tsc and Tx are determined from standard DSC (Differential Scanning Calorimetry) scans at 20° C./min. Tg is defined as the onset temperature of glass transition, Tsc is defined as the onset temperature of super-cooled liquid region, and Tx is defined as the onset temperature of crystallization. Other heating rates such as 40° C./min, or 10° C./min can also be utilized while the basic physics of this technique are still valid. All the temperature units are in ° C. Generally, a larger ΔTsc is associated with a lower critical cooling rate, though a significant amount of scatter exists at ΔTsc values of more than 40° C. Bulk-solidifying amorphous alloys with a ΔTsc of more than 40° C., and preferably more than 50° C., and still more preferably a ΔTsc of 70° C. and more are very desirable because of the relative ease of fabrication.
Another measure of processability is the effect of various factors on the critical cooling rate. For example, the level of impurities in the alloy. The tolerance of chemical impurities, such as oxygen, can have a major impact on the critical cooling rate, and, in turn, the ready production of bulk-solidifying amorphous alloys. Amorphous alloys with less sensitivity to such factors are preferred as having higher processability.
Although a number of different bulk-solidifying amorphous alloy formulations have been disclosed based on these principals, none of these formulations have been based on Au. Accordingly, a need exists to develop Au-based bulk solidifying amorphous alloys capable of use as precious metals.